Field of the Invention
The present invention relates generally to construction of a permanent magnet rotor for use in high speed permanent magnet machines, and more particulary to a permanent magnet rotor construction utilizing high magnetic energy permanent magnets to produce a rotor design having alternating low reluctance and high reluctance poles to thereby allow variable flux linkage, while minimizing axially directed parasitic fluxes and providing superior thermal degradation characteristics, as well as excellent rotor stiffness to allow applications at very high speeds.
The recent development of high energy product permanent magnets provides a great potential for improvement in permanent magnet machine design and efficiency. Increased energy product permanent magnets made of samarium-cobalt currently have an energy product of as high as 26 mega-gauss-oersted (MGO) while permanent magnets made of neodymium-iron-boron have an energy product of as high as 35 MGO, with potential energy products of 45 MGO likely being available in the near future.
Conventional permanent magnet rotors consist of a plurality of axially extending permanent magnets mounted around the outer periphery of a ferromagnetic yoke or core, with damper bars made of electrically conductive, non-magnetizable material being used essentially as spacers between the plurality of permanent magnets arranged about the outer periphery of the yoke. Additionally, a pair of damper end rings, which are also made of non-magnetizable material, may be used at the ends of the yoke to restrain the permanent magnets and damper bars from axial movement on the core. A thin cylindrical hoop, also made of non-magnetizable material, is then installed about the outer periphery of the rotor to retain the permanent magnets and the damper bars in position on the ferromagnetic yoke, particularly against large centrifugal forces operating during high speed rotation of the rotor.
The hoop is generally installed on the rotor by a heat shrinking operation, in which the rotor is cooled and the hoop is heated to a high temperature to cause the hoop to expand relative to the rotor. The hoop is placed over the rotor, and shrinks to form an interference fit with the rotor. Unfortunately, during this operation the magnet surface areas are exposed to abusive surface heat which will cause thermal degradation of the permanent magnets.
While conventional magnets are only slightly susceptible to thermal degradation caused by heat shrinking the hoop over the rotor, the new high energy product permanent magnets are somewhat more susceptible to thermal degradation. Samarium-cobalt permanent magnets may be damaged by heating during the installation of the hoop onto the rotor, resulting in likely degradation in magnetic performance of the permanent magnets, as well as in the likelihood of increasing in a random fashion parasitic fluxes further deteriorating rotor performance. Neodymium-iron-boron magnets are even more sensitive to thermal degradation than are samarium-cobalt permanet magnets, roughly by a factor of three.
It is therefore apparent that it is highly desirable that a permanent magnet rotor utilizing such high energy product permanent magnets be designed to be less susceptible to thermal degradation caused by diffusion of surface heat from the hot retaining hoop to the magnets. While in some instances, it may be possible to install the hoop without using heat shrinking, there are nevertheless many instances in which heat shrinking is the only operation which is practical, and accordingly the redesign of the rotor configuration to make the permanent magnets less susceptible to thermal degradation is an object of the present invention.
It is also desirable to have the best possible magnetic homogeneity of the magnets used in a rotor, since nonuniformity of magnets will cause equalizing fluxes to flow in the direction of the rotor axis, a particularly objectionable phenomenon. The result of this phenomenon is that axially directed parasitic fluxes will exist in the stator core, which is typically made of laminated iron, with the parasitic fluxes being in a direction normal to the laminations. This phenomenon will cause disproportionate losses substantially the same as if the parasitic fluxes were induced in solid iron rather than in a laminated iron stator core.
When using high energy product permanent magnets such as those described above, the magnets are frequently a bonded assembly of smaller pieces, since it is generally possible to magnetize only fairly small pieces of magnetic medium to the high levels of magnetic energy utilized in such magnets. It is therefore apparent that unless the machine is fairly small, the magnets will be a bonded assembly of smaller pieces not having as good magnetic homogeneity as if the magnet were made of a single piece. This factor also will increase the tendency to have axially directed parasitic fluxes. Accordingly, it is an object of the present invention to minimize to the extent possible such axially directed parasitic fluxes by obtaining the best possible magnetic homogeneity of the permanent magnets.
The use of high energy product permanent magnets may make it desirable at times to vary the flux linkage between the rotor and the stator, as described in copending patent application No. 810,968, filed concurrently with the present application, which application is hereby incorporated herein by reference. In order to simplify the scheme by which the flux linkage between the rotor and stator is varied, it has been found that it is desirable to have a rotor having alternating high reluctance and low reluctance poles. Accordingly, it is also an object of the present invention to provide a permanent magnet rotor which has alternating high and low reluctance poles to facilitate a variable flux linkage between the rotor and stator.